A food processing apparatus of the type initially referred to is known, for example, from DE-A 34 08 693. In this food processing apparatus, the electric motor vertically disposed in the motor housing operates via a gear arrangement comprised of two gears and a toothed belt to drive a processing tool adapted to be coupled to a driven shaft and rotating in a working vessel, thus cutting, mixing, kneading, etc. The food material contained in the working vessel, depending on the type of processing tool employed. While the first gear which conventionally has the smaller diameter is connected with the drive shaft of the electric motor in a non-rotating relationship, the second gear is mounted on the driven shaft receiving the processing tool. To produce cooling air, air-directing blades are provided on the second gear which combine with that gear to form the impeller. In this arrangement, the impeller produces a greater or lesser amount of cooling air, in dependence upon the rotational frequency of the electric motor, that is, the higher the rotational frequency of the electric motor, the more cooling air is supplied to the electric motor.
It is a generally known fact in electric motors that their rotational frequency drops significantly if high power demands are made on them, while their rotational frequency increases when the torques to be delivered are small. The rotational frequency of the impeller being dependent on the electric motor, the cooling air stream of the impeller diminishes precisely at a moment when high power losses demand increased cooling of the electric motor. Conversely, the impeller delivers very high amounts of air when power losses are low, that is, when the electric motor has a low power output or operates at no-load, although this condition does not call for intense cooling of the electric motor.
As a result of such shortcomings, only very brief operating periods can be achieved at high load torques and low rotational frequencies, because the air stream produced by the impeller does not, as a rule, suffice to cool the electric motor, unless an electric motor with considerably larger dimensions is used, which would significantly add to the cost of the food processing apparatus. On the other hand, when the electric motor runs at no-load speed, the disadvantage results that the motor is cooled to an unnecessarily high degree. This produces very loud running and air noise, particularly at no-load.
From DE 34 30 023 A1 a drilling machine is known in which the drive motor is cooled by an impeller driven by an additional impeller motor. In this disclosure, the rotational frequency and thus the cooling power of the additional impeller motor are controlled in dependence upon the operating temperature of the electric motor by means of a temperature sensor. In this type of control of cooling power, it is a disadvantage that the temperature measured at the measuring point of the temperature sensor may appreciably lag behind the actual temperature of the motor winding. Thus, when the power demanded from the motor rises rapidly, undesired or even hazardous temperature peaks may occur at the windings before these are reduced again by the impeller motor delivering an increased cooling power.
It is therefore an object of the present invention to provide a food processing apparatus for domestic use which eliminates the above-mentioned disadvantages, builds to compact dimensions in spite of relatively high motor power, in which motor noise is reduced considerably, and which affords high-efficiency cooling.